Definition of U.s. states

A U.S. state is any one of the fifty subnational entities of the United States, although four states use the official title "commonwealth". The separate state governments and the federal government share sovereignty, in that an American is a citizen both of the federal entity and of his or her state of domicile. However, state citizenship is very flexible, and no government approval is required to move between states (with the exception of convicts on parole).